A correlation exists between cardiac electrical abnormalities and coronary vascular abnormalities. These abnormalities often coexist with each other. For example, patients who receive an implantable cardioverter defibrillator (ICD) often also have coronary artery disease (CAD). Ischemia is associated with CAD. One definition of ischemia is a reduced blood flow or a localized tissue anemia due to obstruction of the inflow of arterial blood. Ischemia is capable of initiating electrical arrhythmias; and electrical abnormalities are capable of altering hemodynamics and compromising blood flow. Present therapies typically address one abnormality or the other despite this correlation between cardiac electrical abnormalities and coronary vascular abnormalities.
Cardiac electrical abnormalities are addressed by cardiac stimulus devices such as pacemakers and ICDs. These cardiac stimulus devices monitor electrical activity and restore normal function by delivering pacing strength or defibrillation shock strength electrical pulses. These devices provide a sensing function by sensing the electrical function of the heart. That is, these devices are capable of sensing arrhythmias and intervening electrically. Most cardiac stimulus devices use intracavitary leads placed transvenously for sensing, pacing, and shocking. Now, leads are also being placed intravenously. However, conventional cardiac stimulus devices do not monitor the mechanical performance of the heart or otherwise provide a mechanical performance.
Conventional pacemakers often include an electronics assembly housed in a hermetically sealed enclosure, and one or more leads which connect the pacer directly to the heart tissues to be stimulated and sensed. The electronics assembly is able to be implanted in a suitable area of the body, commonly the upper thorax, because of the length of the lead used, which may be 18 to 30 inches long, for example.
One end of the lead connects to the pacer, while the other end of the lead, referred to as the “distal” end, is attached to an interior surface of one of the chambers of the heart, for example. One or more electrodes typically are disposed at the distal end of the lead through which electrical pulses are delivered to the heart at the site of the electrodes and/or from which sensing occurs. During implantation of conventional pacer systems, it is a common procedure for the physician to insert a stiff wire (“stylette”) through the center of the lead and then to “snake” the lead though a predetermined path to the heart. Often the leads are implanted by guiding them through blood vessels into one or more chambers of the heart. The leads typically pass through valves that separate the atrial from the ventricular chambers.
Although leads have been used for many years in conjunction with implanted pacemakers and defibrillators both to stimulate the heart to beat as well as to sense the electrical activity of the heart, the use of leads is not problem free. For instance, the implantation avenues available for leads to be routed to and through the heart may be limited by the lumenal diameter of the vessels leading to the heart or by valves in the heart, and the ability to chronically fix the lead tends to be influenced by the anchorage available (e.g. trabeculas). Further, because the pressures in the right cardiac chambers are markedly lower than the pressures in the left cardiac chambers, it has been preferred to introduce leads into the right side of the heart because of the reduced risk of blood loss. Thus, for these practical reasons, a physician typically only implants the leads in a relatively few preselected sites in the heart. These sites, however, are not necessarily the optimal sites, but are chosen as a compromise between the complications described above and the patient's cardiac problem. Rather than monitoring the electrical activity in the right ventricle, monitoring the left ventricle's electrical activity, for example, might be preferred instead.
Additionally, it is desired for some applications to sense electrical activity at three, four, or more sites in the heart. Some pacers are implanted with four leads permitting sensing at four different sites in the heart. Four leads tend to be difficult to implant as they occupy a relatively large volume in the blood vessels through which they are passed and sometimes have to be steered along circuitous routes. Further, it is becoming increasingly desirable to sense at more locations in or on the heart than is possible with conventional pacer-lead combinations. It would thus be highly beneficial to have a stimulation and sensing system that provides the diagnostic and therapeutic functions provided by conventional cardiac stimulators yet which employs fewer interconnecting leads, than required by conventional devices, or which can function without using any leads.
U.S. Pat. No. 6,141,588, which issued to Cox et al. and is assigned to Intermedics, Inc., relates to a satellite pacing electrode system and is hereby incorporated by reference in its entirety. This satellite pacing electrode system is a system of remote electrodes that communicate with and are controlled by a central unit. The disclosed satellite pacing system uses epicardial electrodes that are in direct contact with myocardium. Epicardial placement of these electrodes at multiple sites often involves open-chest surgery. It would thus be highly beneficial to have a stimulation and sensing system that provides the diagnostic and therapeutic functions provided by epicardial electrodes of Cox et al., yet which involves less invasive surgery.
Coronary vascular abnormalities are addressed by blood flow and blood pressure monitors and stents, for example. However, these device are unable to sense arrhythmias and electrically intervene. Catheter-based blood flow and pressure monitors presently exist for acute measurements. These monitors provide a sensing function by sensing the mechanical performance of the heart. Examples of these catheter-based monitors include Millar catheters, and Swann-Ganz catheters. However, these catheter-based devices cannot be used chronically; that is, they cannot be implanted and used for long durations. Metal stents are placed intravascularly to reopen arteries in balloon angioplasty operations. However, the function of conventional stents is to prevent restenosis, i.e. to prevent the arteries from narrowing or constricting again.
Given the correlation between cardiac electrical abnormalities and coronary vascular abnormalities, it is desired to provide a chronically-implanted cardiac stimulus device with sensing capabilities to determine cardiac electrical abnormalities and coronary vascular abnormalities for improving arrhythmia therapy. More generally, it is desired to provide a chronically-implanted device that is capable of performing one or various combinations of mechanical, electrical, and chemical sensing and to provide a chronically-implanted device that is also capable of providing appropriate therapy, such as electrical or drug therapy, based on an event sensed by the chronically implanted device or by other devices within a system of devices. Additionally, there is a need to provide a chronically-implanted stimulus device that is capable of being implanted in a large number of desirable locations using less invasive procedures.